Supported scavengers

Supported scavengers are reactive species that selectively quench and/or sequester by-products of the reaction or remove any excess starting materials, and are subsequently removed by filtration (Fig. 2.2). These species, which allow hquid-solid phase extraction are also often referred to as sequestering agents or quenching agents . 

In a similar way, many acidic and basic ion-exchange polymers have been used to quench and work-up reactions as an alternative to aqueous conditions. This is especially valuable if the compound in question is moisture sensitive or hydroscopic, or if a rapid quench and work-up procedure is required. 

---

It is worth noting that the use of polymer-supported scavengers offers a perfectly complementary technique to the use of PSRs, and in several cases the two have been successfully employed together [3,9]. 

Fig. 7.10 Polymer-supported scavengers polystyrene-bound diamine (top left), silica-based diamine (top right), Stratosphere Plugs (bottom left), and Synphase Lanterns (bottom right).

Fig. 7.11 Scavenging efficiencies of polymer-supported scavengers (a) polystyrene, (b) functionalized silica, (c) plugs, (d) lanterns] at room temperature (left) and under microwave heating (right).

S = Silica Supported Scavenger Figure 3.7 The process of using scavengers. 

A comprehensive list of the most common appHcations of supported scavenger agents can be found in several reviews [49]. 

Incorporation of Solid-supported Scavengers into Library Synthesis... 

Kaldor [49 i, 55] demonstrated the advantages of applying solid-supported scavengers to the preparation of parallel arrays in a multi-step fashion. In these studies he examined the clean-up of multiple amine alkylation and acylation reactions using a variety of immobilized electrophilic and nucleophilic scavenger reagents including an amine, isocyanate, aldehyde and acid chloride (Tab. 2.1). 

Scheme 2.57 Three component condensation reaction using polymer supported scavenger purification alternatively the trisamine resin could be substituted for purification in this step.

This strategy has previously been referred to as t A nucleophilic polymer is a scavenger of solid-supported scavengers (SSS), polymer- electrophiles and vice versa,... 

Taddei has developed a soluble PEG supported scavenger 53 to capture a variety of nucleophilic functional groups (Scheme 13) [21]. This scavenger was based on an electrophilic dichlorotriazine core and relied on selective precipitation (by the addition of ether to acetonitrile) to remove it from the reaction mixture. This scavenger 53 is particularly versatile, and has been used to remove primary, secondary and tertiary alcohols, diols and thiols... 

Use of a readily separable, polymer-supported reagent to facilitate a chemical transformation is not always a panacea. Yields should preferably be high, and the recovery and purification of the desired product from the reaction mixture should be practical. Polymer-supported scavenging agents to scavenge byproducts have been employed, as also have polymer-supported reagents that capture the desired product from a reaction mixture.64... 

The reagent to be sequestered (CX, Fig. 8.32, path D) may contain a chemical functionality that is not present in the reaction product and can be reacted with a suitable reactive, supported scavenger, forming a covalent bond and again leaving pure product P in solution. Many supported covalent scavengers have appeared recently in the literature (202, 206, 207, 215-222), and some stmctures (8.58-8.63, Fig. 8.33,... 

Figure 8.32 Solid-supported scavengers basic principles.

Figure 8.33 Solid-supported scavengers tagging reagents 8.54-8.57 and supported, covalent...

Figure 8.34 Solid-supported scavengers purification of the P-amino alcohol 8.64 using the covalent scavenger 8.62.

Figure 8.35 Solid-supported scavengers sequestration-enabling reagents 8.65-8.70.

Polymer-supported scavengers for solution-phase library synthesis... 

The adoption of PASP synthesis presents the opportunity for in-Une purification strategies, through the use of supported scavengers or reagents and catch-and-release strategies. In this way, conventional purification techniques such as aqueous workups and column chromatography can be eliminated. In addition, the use of PASP synthesis reduces the vast number of traditional synthetic manipulations to a series of repetitive incubations and subsequent filtrations, which are in principle well suited to automated processing. 

---